Correct documentation errors in regards to UTF8 params

Correct documentation errors in regards to UTF8 params

This fixes numerous bugs in documentation in regards to UTF8
params and their sizes. The returned size should always be without the
terminating NUL byte. On the other hand on the requestor side
the size of the buffer should include the NUL byte if it expects it
being included in the returned string.

Also make this clear in the EVP_PKEY_get_group_name() documentation
which uses utf8 string params under the hood.

Fixes #16287

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16296)